Thermopower and Nernst effect in the Dirac semimetal Cd$_3$As$_2$*

Dirac semimetals and Weyl semimetals are 3D analogues of graphene in which crystalline symmetry protects the nodes against gap formation. Cd$_3$As$_2$ was predicted to be Dirac semimetal [1], and recently confirmed to be so by photoemission [2,3]. Here we report an interesting property in Cd$_3$As$_2$ that was unpredicted, namely a remarkable protection mechanism that strongly suppresses backscattering in zero H. In single crystals, the protection results in ultrahigh mobility $\sim 9\times 10^6$ cm$^2$ V$^{-1}$ s$^{-1}$ [4] at 5 K. Suppression of backscattering results in a transport lifetime 10$^4$ times longer than the quantum lifetime. The lifting of this protection by H leads to a very large magnetoresistance. Quantum oscillations in resistivity, Seebeck and Nernst, show beating effect.We discuss how this may relate to changes to the Fermi surface induced by H. [1] Wang, Z. J. et al. , Phys. Rev. B $\mathbf{88}$, 125427 (2013). [2] Sergey, B. et al. , Phys. Rev. Lett. $\mathbf{113}$, 027603 (2014). [3] Neupane, M. et al. , Nature Commun. $\mathbf{5}$, 3786 (2014). [4] Tian Liang et al. , Nature Materials, in press.